The present invention is directed to an apparatus and a method for monitoring the solidification properties, including the viscoelastic properties, of a solidifiable liquid film (e.g. coating).
The performance of a liquid film, specifically a coating, depends on many variables, such as its viscosity, its elasticity, its surface tension and the rate of solvent evaporation. These variables must be optimized to achieve desirable characteristics, such as flow and leveling, anti-sag, anti-mounding, drying rate, pigment dispersion and other preferred characteristics. For example, after a liquid film is deposited to a substrate, the liquid film needs time to level. However, the liquid film must also dry quickly so as not to sag. Good flow and leveling characteristics often require low viscosity liquid films with Newtonian behavior while good anti-sag requires liquid films having high viscosity at low shear rates. The challenge when forming films is to control these competing properties to provide liquid films with the desired characteristics.
The dryness of a liquid film on a substrate is often described as a surface phenomenon and is defined in many different ways, such as by viscosity. For example, the American Society for Testing and Materials (ASTM) describes the standard test method for measuring times of drying or curing of organic coatings using mechanical recorders in ASTM D5895-01. In this test a coating is applied to a glass strip and a stylus placed on the wet coating and moved either in a linear motion or in a 360° arc at a constant speed. Various drying times, e.g., set-to-touch time, and tack-free time, dry-hard time, and dry-through time, are visually determined by observing the track of the stylus on the glass slide. Unfortunately test results of this kind are quite subjective and difficult to accurately reproduce.
Many other ways have been proposed to measure the dryness or the drying rate of a liquid film. For example, the dry-to-touch time is commonly used and is defined by the time from the application of a liquid film onto a surface to the time the film can be touched with an object (usually the hand of the tester) without the film being transferred from the surface to the object (hand). However, the dry-to-touch time is dependent upon many subjective criteria, such as how much pressure is applied, and is not consistently reproducible.
Another method to determine the drying rate is scratching a coating that has been applied to a surface using a pencil or other sharp object (e.g. Tukon test) at certain elapsed times. Yet another method for measuring the relative drying rate is a gravimetric method that measures the amount of solvent loss from the liquid film.
A drawback to the methods mentioned above is that they only measure properties related to the viscosity of the liquid film. In addition to the viscosity, solidifiable liquid films also exhibit elasticity that changes during the solidification process. A direct measurement of the viscoelastic properties (the viscosity and elastic properties of a liquid film) provides more accurate and detailed information of the solidification process of a liquid film.
A frequently used technique to monitor the viscoelastic properties of a coating deposited on a substrate is the torsion braid analysis (J. Polymer Sci. Part C, No. 35, pp 3-21 (1971)). The torsion braid analysis monitors decaying amplitude and frequency to determine the affect of a liquid film as it solidifies on a substrate. One problem with this analysis is that the restoring force or the elastic modulus of the substrate is so much stronger than that of the liquid film that it is very difficult to determine the relatively small effect of the liquid film on the total torque. In addition, this test is sensitive to such variables as non-uniform distribution of the coating on the substrate and irregular drying patterns, which reduce the accuracy and reproducibility of the test.
German patent DE 19806905 provides another technique for measuring changes in the viscoelastic properties of a liquid film as it solidifies on a thin hard plate. The technique disclosed in DE 19806905 measures the viscoelastic properties by measuring the resonance frequency applied either in a torsional or flexural mode using a substrate, such as a rectangular plate, coated with a liquid film. The resultant force and the resonance frequency shift that occur as the liquid film dries are measured by a piezoelectric material and converted into loss (G″) and storage (G′) moduli.
U.S. Pat. No. 4,799,453 and Japanese patent 63145943 describe apparatus that monitor film formation during curing by UV, oxidation, and evaporation of solvents. In these patents the solidification characteristics of the liquid film are electronically monitored by measuring the resistance to movement of a stylus through the liquid film as it solidifies. Although this technique improves the accuracy as compared to other techniques, such as the swab resistance test or testing the film surface with one's thumb, it monitors changes only in the hardness of the film as it dries.
German patent DE 3420341 teaches a technique to measure the drying time of a liquid film by a rotating an object, such as a ball, on an inclined disc. The position of the ball placed eccentrically on a coated disc is monitored by a light detector. The inclined disc is rotated by a motor to keep the ball at the same position on the disc. The rotational speed of the disc necessary to maintain the ball in a stationary position is proportional to the viscosity of the liquid film. The viscosity is plotted as a function of time to provide a viscosity verses time measurement for the liquid film. In addition to requiring intricate and expensive equipment, this method only measures the viscosity or stickiness of the liquid film and does not measure other solidification properties, such as elasticity of the liquid film.
In view of the above, it is apparent that there is a need for a means of reliably, consistently and accurately measuring the solidification properties of a liquid film on a substrate under laboratory conditions.